Three vendors have released fixes for vulnerabilities found in the critical firmware used during a computer’s startup, according to an advisory from the U.S. Computer Emergency Readiness Team.

The vulnerabilities could allow an attacker to bypass a feature called Secure Boot, which verifies that firmware components carry a correct digital signature ensuring the software’s authenticity. The attacker could then replace the device’s firmware.

The flaws lie within some UEFI (unified extensible firmware interface) systems, the advisory said. UEFI is a firmware interface that was designed to improve upon BIOS.

A boot script within the UEFI S3 Resume path “resides in unprotected memory which can be tampered with by an attacker with access to physical memory,” the advisory said.

An authenticated local attacker could bypass Secure Boot and reflash, or replace, the firmware even if signed firmware updates are supposed to be used. An attack could also cause a system to be inoperable.

Several vendors have taken action. American Megatrends Incorporated (AMI), which makes BIOS and UEFI firmware, has “addressed the issue on a generic basis and is working with OEMs to implement fixes for projects in the field and production.”

Intel and Phoenix Technologies, which also makes UEFI software, have issued fixes, the advisory said.

The advisory was one of three issued by U.S. CERT on Monday. The agency also warned of a “race condition” vulnerability in some Intel chipsets that could allow the bypass of a BIOS locking mechanism, allowing malicious code to be inserted into firmware.

American Megatrends and Phoenix Technologies have issued updates to address the issue, but it’s unknown if other major vendors may be affected, according to the advisory.

U.S. CERT also warned in a third advisory of a buffer overflow in the open-source EDK1 project’s UEFI reference implementation. One affected vendor that uses the firmware, Insyde Software, has fixed the issue.

American Megatrends, Apple, IBM, Intel and Phoenix Technologies are not affected by that flaw. However, it’s not known whether other large vendors may be vulnerable.